A motor driven valve adapted to selectively open or close the valve by rotating the valve body thereof based on driving by a motor has been conventionally known, and the motor driven valve of this type is generally so arranged that, while the motor is shut down, the rotary shaft of the motor is fixed so as not to be rotatable by a brake mechanism provided inside said motor for preventing the valve body of said valve from being rotated by the pressure of a fluid passing through a flow passage.
Therefore, if it becomes impossible to control the valve for opening and closing due to the inability to pass electric current through the motor due to a power failure or some other troubles, the valve body of the valve remains in the state prior to suspension of the energy supply, and thus, when energization becomes impossible and the valve body is left in the opened state, there has been a possibility that a tank may undersirably overflow. Accordingly, the conventional practice is such that an operator has to manually close the valve at the time as the trouble occurs, but such a practice is not necessarily satisfactory from the viewpoint of security, because it is almost impossible to control a plurality of valves in a short period of time, and thus, there has been a case where some valves are left unclosed desirably.
For eliminating the disadvantages as described above, another arrangement has also been proposed which is adapted to automatically close the valve by changing-over by means of a magnet clutch through utilization of a restoring force of a spiral spring manually wound in advance. The above prior art device, however, not only requires troublesome procedures for the preliminary manual winding of the spiral spring, but also involves a possibility that the spiral spring may be left unwound carelessly, and moreover, since it is necessary to adopt an expensive magnet clutch, the manufacturing unit cost tends to be excessively high, and therefore, the arrangement has not been placed into actual applications as yet.